Electronic structures including a conductive layer structure are known. A conductive layer structure serves, for example, as a cathode contact in transparent OLED (organic light-emitting diode) components. Such an application usually requires two transparent electrodes, for example a transparent ITO (indium tin oxide) anode for hole injection and a transparent Ag (silver) cathode for electron injection. It is known for the Ag cathode to be thermally vapor-deposited onto organic electron transport layers as an Ag film by means of PVD (physical vapor deposition), for example.
According to N. Kaiser et al., Applied Optics, Vol. 41, Issue 16, pp. 3053-3060 (2002), the growth of thin metal films can be characterized by the formation of nuclei and by the growth behavior. In this case, depending on the interatomic interactions between surface atoms on the interface of a surface species to be coated with a metal film and so-called adatoms, the progressively absorbed atoms of the metal film on the surface atoms of the surface to be coated, to a first approximation, a distinction is made between three growth models:                In accordance with the Frank-van der Merwe model (layer growth), new layers grow homogeneously monolayer by monolayer on a surface. The interatomic interaction between surface atoms and adatoms is greater than the interaction between adjacent adatoms. A surface which fulfils this condition is a good so-called adhesion promoter for a given adatom.        In accordance with the Volmer-Weber model (island growth), the adatoms grow in an island-like fashion. The interatomic interaction between adjacent adatoms is greater than that between adatoms and surface atoms.        In accordance with the Stranski-Krastanov model (layer growth and island growth), the interatomic interaction between the surface atoms and the adatoms and thus the adhesion on the first monolayer of the adatoms is initially higher than on the pure surface. As a result, at least one completely closed monolayer forms initially. Starting from a critical layer thickness, island-like growth upward takes place, since the interatomic interaction between the adjacent adatoms increases with the layer thickness of the metal film.        
The three growth models can be described, to a first approximation, thermodynamically via the interplay between contact angle and surface energies by means of what is referred to as Young's equation.γB=γ*+γA cos φ  (1)
In this case, γB denotes the surface energy of the surface species to be coated, γA denotes the surface energy of the metal film to be applied, γ* denotes the interfacial energy between the surface to be coated and the metal film surface, and φ denotes the contact angle between γA and γ*.
For the Frank-van der Merwe model, it holds true that for every position φ=0 and thereforeγB>γ*+γA  (2)
For the Volmer-Weber model, it holds true that for every position (homoepitaxial systems) φ>0 and thereforeγB<γ*+γA  (3)
For the Stransky-Krastanov model, it holds true that firstly φ=0 and thusγB>γ*+γA  (2)and finally, starting from a critical position, if the interfacial energy γ* also increases as the layer thickness increases, φ>0 and thusγB<γ*+γA  (3)
It is known that metal films in heteroepitaxial systems grow on surfaces in principle in an island-like fashion in accordance with the Volmer-Weber model.
Such islands of the adatoms on the surface are initially transparent to radiation in the visible wavelength range and in the infrared range. Upon reaching the so-called percolation layer thickness, thus the layer thickness at which the islands have grown in such a way that they touch one another, the metal films form continuous layer structures. This process is also referred to as coalescence. The layer structures of the metal films are initially transparent to radiation in the visible wavelength range and reflect in the infrared range. As the layer thickness increases, however, the radiation in the visible wavelength range is also reflected; therefore, the transparency of the metal film decreases significantly as the layer thickness increases.
If there are pronounced interatomic interactions between the adjacent adatoms, the island growth is also additionally fostered by intrinsic thermal diffusion. Such thermal diffusion processes can be described by means of the Arrhenius equation.D=D0*exp[−ΔE/kT]  (4)
In this case, D denotes the diffusion coefficient at a given temperature T (in kelvins), D0 denotes the so-called pre-exponential factor, k denotes the Boltzmann constant, and ΔE denotes the activation energy necessary for the diffusion process.
For a sufficiently low temperature, it holds true that ΔE>kT, and the activation energy for the diffusion process (ΔE) is thus greater than the thermal energy of the surface (kT); this makes the diffusion more difficult and the adatoms adhere to their adsorption sites on the surface.
However, if it holds true that ΔE<kT, and if the activation energy for the diffusion process (ΔE) is thus less than the thermal energy of the surface (kT), then the adatoms can move relatively freely on the surface; this is referred to as increased diffusion mobility in this case. In this case, the probability of a collision of two adatoms is higher than for the case where ΔE>kT. If two adatoms collide with one another, then a dimer can arise which remains as a stable nucleation nucleus; such nuclides are nuclei for island formation and therefore additionally foster island growth.
In this case, what should be regarded as disadvantageous about the known electronic structures including a conductive layer structure is the circumstance that organic layers suitable for organic light-emitting diodes (OLEDs), for example, are often poor adhesion promoters for thin metal films, for example for thin Ag cathode films. The island growth exhibited by metals is additionally fostered thereby. Such island growth limits the lateral conductivity of the Ag cathode films and prevents efficient homogeneous electron injection. This in turn can disadvantageously affect the component performance, in particular the efficiency of an OLED, for example. It is only as the layer thickness increases that the islands combine (coalescence) and the lateral conductivity increases starting from the so-called percolation layer thickness. However, if the layer thickness increases, absorption and reflection processes for the visible wavelength range are increasingly fostered and the transparency of an OLED, for example, decreases significantly.
This can be attributed to a relatively high interatomic interaction between two adjacent adatoms, for example silver (Ag), and, in comparison therewith, lower interatomic interactions between an adatom and a surface atom, for example of an organic compound. For a pronounced interatomic interaction between an adatom and a surface atom, illustratively a relatively high surface energy γB of the surface species is provided. The surface energy γB of the atoms of the surface species increases material-specifically with the bond energy of the respective atoms. The bond energy of the atoms is proportional to the respective specific sublimation temperature. However, OLED-suitable organic surfaces, in particular, are generally distinguished precisely by relatively low sublimation temperatures and accordingly relatively low molecular bond energies and thus by low surface energies YB.
Various conventional electronic structures include a transparent conductive layer structure. In this regard, a description is given of so-called stacked electrode concepts in the system gold (Au)-aluminum (Al). In this regard, U.S. Pat. No. 7,796,320 B2 describes stacked electrodes, for example of the composition Al/Au, Au/Al/Au, Al/Cu, Cu/Al/Cu, Cu/Ag, Ag/Cu, Au/Ag, Ag/Au, Ca/Ag, Ag/Ca and Cr/Au. From the compositions described in U.S. Pat. No. 7,796,320 B2, the three-layer construction Au/Al/Au is described as the stack compound having the best transmission properties. However, what is problematic about the construction of the layer structure as described in U.S. Pat. No. 7,796,320 B2 is the high complexity of the three-layer structure of the stacked electrode and the high production outlay associated therewith.
C. J. Lee et al., R., Appl. Phys. Lett. 89 (2006), 123501, describe, for example, a top emitting OLED component in which a layer of barium (Ba) having a layer thickness of 10 nm was deposited thermally on a 5 nm thick 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layer. What is problematic, however, is that barium (Ba) is an electrode material which reacts very sensitively to moisture and oxygen, as a result of which the usability and lifetime of a corresponding OLED component, for example, is significantly reduced.
R. B. Pode et al., Appl. Phys. Lett. 84 (2004), 4614-4616, investigated double-layer metal cathodes for top emitting OLED components. Thermally vapor-deposited calcium (Ca), magnesium (Mg), or lithium fluoride (LiF) in each case in combination with silver (Ag) and aluminum (Al) were investigated in this case. A high transmission in conjunction with a relatively low sheet resistance was reported for layers of 10 nm calcium and 10 nm silver. However, calcium is an electrode material which reacts very sensitively to moisture and oxygen, as a result of which the usability and lifetime of an electronic component, for example of a corresponding OLED component, is significantly reduced.
S. Y. Kim et al., Thin Solid Films 517 (2009), 2035-2038 describe semitransparent cathodes having the layer sequence strontium (Sr) (having a layer thickness of 8 nm to 10 nm)/Ag (10 nm), which were thermally vapor-deposited. In this case, Sr was used as a so-called wetting promoter for improving the growth properties of the Ag film. A relatively high transmission in conjunction with a relatively low sheet resistance was achieved in this case, too. However, the percolation layer is attained only at relatively high layer thicknesses of the Ag film, the film not being completely closed or even coherent even at a layer thickness of 20 nm silver (Ag). However, at this Ag layer thickness, absorption and reflection processes for the visible wavelength range are already significantly fostered and the transparency of an OLED, for example, decreases significantly. Consequently, Sr is not suitable as a wetting promoter particularly for transparent electronic applications.
Furthermore, G. Gu, V. Bulovic et al., Appl. Phys. Lett. 68 (1996) 2606-2608; P. E. Burrows et al., J. Appl. Phys. 87 (2000), 3080; L. S. Hung et al., Thin Solid Films 410 (2002), 101), described thermally vapor-deposited, semitransparent metal films having low work functions to which transparent conductive oxides (such as e.g. ITO) were finally applied by means of sputtering techniques. However, application of electrode layers by sputtering, without further buffer layers, can lead to damage to the underlying layers, and thus requires additional complex production steps and therefore significantly increases the production outlay.
U.S. Pat. No. 6,794,061 B2 describes magnesium (Mg) cathodes vapor-deposited onto a wetting promoter layer. It is apparent that Mg cathodes have poor adhesion properties on organic layers. Al:Mg or Ag:Mg alloys made it possible to improve the wetting and to deposit more homogeneous layers. However, pure Mg cathodes have the advantage of low work functions (˜3.7 eV) that are no longer attained by means of the alloys. All metals or metal compounds of main groups 1 to 15 of the periodic system having atomic numbers of greater than or equal to 19 were taken into account as wetting promoters. However, magnesium (Mg) is an electrode material which reacts very sensitively to oxygen and, moreover, is very highly inflammable. As a result, the usability and lifetime of an electronic structure, for example of an OLED component, is significantly reduced and production is significantly more complex.
Germanium (Ge) as a wetting promoter was described for silver atoms on inorganic silicate or SiO2 surfaces in Weiqiang Chen et al., OPTICS EXPRESS 18 (2010), 5124; and in Logeeswaran V J et al., NANOLETTERS 9 (2009), 178-182. In this regard, a homogeneous coating without pronounced island formation of the silver atoms is achieved according to Weiqiang Chen et al. by virtue of the fact that the bond energy between Ag—Ge is higher than for Ag—Ag (Ag—Ge ΔH=174.5±21 kJ/mol and Ag—Ag ΔH=162.9±2.9 kJ/mol). Higher bond energies reduce surface diffusion processes and thus island-like Volmer-Weber growth. In this regard, Logeeswaran V J et al. report that the activation energy for a surface diffusion of Ag atoms on Ge surfaces is 0.45 eV, while it is only 0.32 eV on SiO2 surfaces. Consequently, Logeeswaran V J et al. show that a germanium film a few monolayers thick on the SiO2 surface fosters large-area closed Frank-van der Merwe growth of thin Ag films. However, the investigations are restricted exclusively to inorganic SiO2 surfaces. However, as an insulator, SiO2 is unsuitable as conductive surface species in electronic structures, in particular as electrode surface species for electrically conductive structures, and is thus not suitable for an application in an electrically active region. Moreover, no restricted production conditions for film deposition corresponding to organic surfaces hold true for insensitive inorganic SiO2 surfaces.
For OLED applications, the transparent Ge/Ag layer sequence with an Ag film closed over a large area by the germanium nucleation has been described hitherto only as an anode concept likewise on borosilicate glass surfaces (P. Melpignano, C. Cioarec, R. Clergereaux, N. Gherardi, C. Villeneuve, L. Datas, Organic Electronics 11 (2010), 1111-1119). In this case, on borosilicate glass, a 5 nm thick germanium nucleation film and finally a 25 nm thick semitransparent Ag film as anode were thermally vapor-deposited. Finally, a blue OLED with bottom emission was deposited on said anode. However, the electrode described is only semitransparent and in turn deposited on an SiO2 or borosilicate glass surface, which in turn, as an insulator, is unsuitable as a surface in active regions of electronic structures.
Typical metal films, for example silver films (also called Ag films hereinafter), of electronic structures according to the related art which were applied to a surface are therefore completely closed only at relatively large layer thicknesses. However, an increasing layer thickness significantly limits the transparency of the electronic structure. Other proposals are very complex in terms of production and characterized by complicated layer constructions or have only a very limited lifetime and/or usability and/or component performance in electronic structures on account of disadvantageous material properties.
A conceivable increase in the deposition rate of, for example, the PVD process used for application for depositing the respective metal film on the respective surface, which makes it possible to produce a multiplicity of nucleation nuclei on the surface already in the first monolayers, is possible only to a very small, insufficient extent, however, taking customary production conditions into account.
Reducing the temperature of the substrate surface in order to reduce the diffusion processes on the substrate surface is also possible only to a very limited and therefore insufficient extent taking customary production conditions into account.